Testing Soil

Lynda Ellis, UM Extension Master Gardener in Anoka County, August 25, 2015.

We add water and fertilizer to our soil. Are we adding the right amounts and types of fertilizer? Let’s have our soil tested to find out. Several companies can test soil, but the University of Minnesota is the only organization in the state that will test residential soil from homeowners. Most yards have soil in several different areas; the two most important for us to test are found in lawns and gardens. While samples for a soil test can be taken anytime the soil is not frozen, the most common time is just before fertilizer application, in the spring or fall.

The test is only as good as the sample being tested. Dig five to ten holes in the area (lawn or garden) you want to test. Dig 6” deep in gardens, 3” deep in established lawns. Scrape off the grass or surface mulch or litter in each divot and put the dirt into a bucket. Mix them together, and take about 2 cups of this mix into a sample bag or box. If you want results for multiple areas in your landscape, sample each area separately and send in multiple samples. Label each sample, so you will remember where it came from.

Samples are sent to the University of Minnesota Soil Testing Laboratory, 135 Crops Research Building, 1902 Dudley Ave., St. Paul, MN 55108. Sample submission forms and other soil testing information are found at http://soiltest.cfans.umn.edu/.

Each test costs $17. In 7-10 business days, you will get a report with the amounts of phosphorous (P), potassium (K), pH, and organic matter in the sample, and recommendations for fertilizer. The soil test will not say if you have disease-causing organisms in your soil, plant-attacking insects or nematodes, herbicide residues that might be harmful to your plants, or beneficial microorganisms.

Look at the pH number. Most garden plants like soil pH between 6.0-7.0. Most lawn grasses like it between 5.5-6.5. A list of plants that prefer different pH ranges, and instructions on how to change soil pH, are found at the link at the end.

Next look at the fertilizer recommendations. They give you the amount in pounds of N (nitrogen), P (phosphorous) and K (potassium) to apply to a 1,000 square foot lawn or 100 square foot garden. Most Minnesota soils have sufficient P; add additional P only if starting a new lawn, or if the recommendations indicate it is needed. It may be hard to find an exact match to your recommendations in the fertilizers sold in garden centers; it is most important to match the N (nitrogen) requirement.

With a little time and effort, you can receive exact fertilizer and other recommendations for your yard. For more information, including example fertilizer calculations, see: http://www.extension.umn.edu/garden/yard-garden/soils/soil-test-interpretations-and-fertilizer-management/.